Abstract
Near-ternary eutectic Sn-Ag-Cu alloys are leading candidates for Pb-free solders. These alloys have three solid phases: β-Sn, Ag3Sn and Cu6Sn5. In order to reveal the solidification process of the alloy, the technique for quenching samples during freezing was used to evaluate the freezing characteristics of near eutectic ternary Ag-Cu-Sn alloys. In the present study, Sn-3.5Ag-0.5Cu and Sn-3.5Ag-1.5Cu (mass%) samples were investigated. These samples were subjected to the thermal program to give a cooling rate of about 0.05°C/s.
In the case of Sn-3.5Ag-0.5Cu alloy, the solidification structure almost corresponded with the estimation from equilibrium phase diagram, though the solidification process was different. In this study the Sn-Ag3Sn eutectic nucleated immediately after primary β-Sn solidified. On the other hand, eutectic Cu6Sn5 did not nucleate easily, though the temperature of the specimen indicated at 217°C (ternary eutectic temperature).
In the case of Sn-3.5Ag-1.5Cu alloy, the solidification structure as well as solidification process were quite distinct from the estimation of equilibrium phase diagram. Primary Cu6Sn5 was the first phase formed in the liquid. This was followed by the nucleation and growth of psud-primary Ag3Sn. The recalescence was observed when β-Sn nucleated and grew as a halo. The ternary eutectic structure emerged after Sn-Ag3Sn binary eutectic solidification.
